The royals were greeted in Harlow by Vice Lord Lieutenant of Essex Jonathan Douglas-Hughes, the Queen's representative in the county.
But the 72-year-old, who was wearing full ceremonial uniform including a sword and boots with spurs, tumbled backwards over a concrete bollard shortly after the couple got out of their car.
William rushed to help the stricken official, who got up looking somewhat embarrassed. Mr Douglas-Hughes turned to William and said 'Sorry about that' and the Duke replied 'No, it's all right'.
William and Kate were arriving at a windswept Stewards Academy in Harlow, Essex, to discuss how young people cope with issues such as starting a new school.